1. Field of the Invention
The present invention relates to an automobile antenna and more particularly to an improved antenna system which can efficiently detect surface currents induced on the body of an automobile by received broadcast waves with detected signals being supplied to various built-in receivers mounted within the vehicle body.
2. Description of the Prior Art
Modern automobiles require antenna systems for causing built-in receivers to positively receive various TV and radio broadcast waves or communication waves for car-telephones. Such antenna systems also are very important for citizen-ban communications between automobiles and other movable or ground stations.
In the prior art, a pole type antenna was generally known which extends outwardly from the body of an automobile. Although the pole antenna exhibits a preferable performance on receiving broadcast and communication waves, it is always an obstruction from the standpoint of the design of the vehicle body.
Such a pole antenna tends to be carelessly or intentionally damaged or to be stolen. Furthermore, the pole antenna tends to produce an unpleasant noise when an automobile, on which the pole antenna is mounted, runs at high speeds.
Since the frequency bands of broadcast or communication waves have recently been increased, the automobile requires a plurality of antennas compatible with the number of the frequency bands. This further degrades the aesthetic appearance of the automobile. Furthermore, there results the problem that the reception performance is diminished by electrical interference between the antennas.
Some attempts have been made to eliminate or conceal the pole antennas. One of the attempts provides an antenna wire applied to the rear window glass of the vehicle body.
Another attempt is that surface currents induced on the vehicle body by broadcast waves are detected. It apparently appears that the utilization of such surface currents is most positive and efficient. However, experiments showed negative results.
One of the reasons why the surface currents induced on the vehicle body by the broadcast waves could not advantageously be utilized is that the level of the surface currents is not as large as expected. The prior art intended mainly to utilize surface currents induced on the roof panel of the vehicle body. Nevertheless, outputs could not the detected at sufficient levels.
The second reason is that a very large proportion of noise is present in the surface currents. The noise is primarily from the ignition and regulator systems of an engine and cannot be eliminated as long as the engine is running.
Some proposals have been made to overcome such problems. One of the proposals is disclosed in Japanese Patent Publication Sho No 53-22418 in which an electrical insulator is provided in a current concentrating portion on the vehicle body, with the level of current being detected by a sensor at the opposite ends of the insulator. Such an arrangement is effective to detect practicable signals which are supeior in SN ration. However, for example, a cut-out must be formed in a portion of the vehicle body to accomodate a pick-up. This is not suitable for normal mass-production of automobiles.
Japanese Utility Model Publication Sho No 53-34826 shows another proposal which provides an antenna including a pick-up coil for detecting currents on a pillar of the vehicle body. Such an arrangement is advantageous in that the antenna can be concealed within the vehicle body. In fact, however, it is not practicable that the pick-up coil must be disposed adjacent to the pillar of the vehicle body in a direction perpendicular to the length of the pillar. Furthermore, such an arrangement does not provide a pick-up device which can obtain practicable outputs of the antenna. It appears that this proposal is only an idea.